1. Gas Laws: Pressure, Volume, and Hot Air
Chapter 12

2. Opening thoughts… Have you ever: Seen a hot air balloon?
Had a soda bottle spray all over you?
Baked (or eaten) a nice, fluffy cake?
These are all examples of gases at work!

3. Properties of Gases
You can predict the behavior of gases based on the following properties:
Pressure
Volume
Amount (moles)
Temperature
Lets review each of these briefly…

4. Pressure
Pressure is defined as the force the gas exerts on a given area of the container in which it is contained. The SI unit for pressure is the Pascal, Pa.
If you’ve ever inflated a tire, you’ve probably made a pressure measurement in pounds (force) per square inch (area).

5. Pressure Units KEY UNITS AT SEA LEVEL 101.325 kPa (kilopascal) 1 atm
760 mm Hg
760 torr
14.7 psi

6. Volume
Volume is the three-dimensional space inside the container holding the gas. The SI unit for volume is the cubic meter, m3. A more common and convenient unit is the liter, L.
Think of a 2-liter bottle of soda to get an idea of how big a liter is. (OK, how big two of them are…)

7. Amount (moles)
Amount of substance is tricky. As we’ve already learned, the SI unit for amount of substance is the mole, mol. Since we can’t count molecules, we can convert measured mass (in kg) to the number of moles, n, using the molecular or formula weight of the gas.
By definition, one mole of a substance contains approximately x 1023 particles of the substance. You can understand why we use mass and moles!

8. Temperature
Temperature is the measurement of heat…or how fast the particles are moving. Gases, at room temperature, have a lower boiling point than things that are liquid or solid at the same temperature. Remember: Not all substance freeze, melt or evaporate at the same temperature.
Water will freeze at zero degrees Celsius. However alcohol will not freeze at this temperature.

9. Temperature (cont.) ºF ºC K -459 32 212 -273 100 273 373
Always use absolute temperature (Kelvin) when working with gases. ºF ºC K
-459 32
212
-273
100
273
373

10. STP: you need to memorize this
Standard Temperature & Pressure
0°C K
1 atm or kPa

11. How do they all relate?
Some relationships of gases may be easy to predict. Some are more subtle. Now that we understand the factors that affect the behavior of gases, we will study how those factors interact.

12. Boyle’s Law
This lesson introduces Boyle’s Law, which describes the relationship between pressure and volume of gases. P V
P1V1 =
P2V2

13. Boyle’s Law
This law is named for Robert Boyle, who studied the relationship between pressure, p, and volume, V, in the mid-1600s.
Boyle determined that for the same amount of a gas at constant temperature, results in an inverse relationship: when one goes up, the other comes down.
pressure
volume

14. Charles’ Law
This lesson introduces Charles’ Law, which describes the relationship between volume and temperature of gases. V1 T1 = V2 T2 V T

15. Charles’ Law
This law is named for Jacques Charles, who studied the relationship volume, V, and temperature, T, around the turn of the 19th century.
This defines a direct relationship: With the same amount of gas he found that as the volume increases the temperature also increases. If the temperature decreases than the volume also decreases.
volume
temperature

16. P1 T1 = P2 T2 Gay-Lussac’s Law P T
The pressure and absolute temperature (K) of a gas are directly related at constant mass & volume. P1 T1 = P2 T2 P T

17. What does it mean?
For a gas at constant mass and volume, the pressure and temperature are directly related.
temperature
pressure

18. P1V1 T1 = P2V2 T2 P1V1T2 = P2V2T1 Combined Gas Law
It is a law that combines the previous laws into one.
P1V1 T1 =
P2V2 T2
P1V1T2 = P2V2T1

19. V1 n1 = V2 n2 Avogadro’s Principle V n
Equal volumes of gases contain equal numbers of moles
at constant temp & pressure
true for any ideal gas V n V1 n1 = V2 n2

20. What does it mean?
For a gas at constant temperature and pressure, the volume is directly proportional to the number of moles of gas.
volume
pressure

21. Dalton’s Law of Partial Pressures
The total pressure of a mixture of gases equals the sum of the partial pressures of the individual gases.
Ptotal = P1 + P

22. UNIVERSAL GAS CONSTANT
Ideal Gas Law
UNIVERSAL GAS CONSTANT
R= Latm/molK
R=8.315 dm3kPa/molK PV =
nRT